1. Field of the Invention
The present invention relates to a structure applied to a photolithographic process. More particularly, the present invention relates to a structure applied to a photolithographic process for improving critical dimensions uniformity.
2. Description of the Related Art
In the fabrication of a semiconductor device, a number of photolithographic processes needs to be carried out. Since each photolithographic process is going to affect the final quality of the semiconductor device, photolithography is a very important process. For example, accuracy of the photolithographic process is a major factor that determines the highest possible circuit density and the ultimate reliability of an integrated circuit. Furthermore, the photolithographic process affects the positioning and uniformity of metallic interconnects and via plugs connection with transistor significantly.
However, in a conventional photolithographic process, the photoresist layer can hardly absorb all light emitted from the photo-exposure light source. Consequently, a portion of the incoming light will penetrate through the photoresist layer and reflect from the substrate. The incoming light may interfere constructive or destructively with the reflected light to produce standing waves. Under such circumstances, the profile of the photoresist layer after photoresist patterning will be fuzzy.
To resolve the back reflection problem, an anti-reflection coating is formed underneath the photoresist layer (that is, form an anti-reflection layer between the photoresist layer and an underlying film layer) to absorb the light that penetrates through the photoresist layer during a photo-exposure. In the presence of the anti-reflection coating, interference between incoming and reflected light is minimized. In general, the anti-reflection coating is fabricated using a dielectric material such as silicon nitride, silicon oxynitride or an organic material with high light absorption properties.
However, the light absorption coefficients of all these materials are often insufficiently high to absorb most of the incoming light. In other words, a portion of the incoming light still penetrates through the anti-reflection coating and underlying film layers and gets reflected by the substrate surface to interfere with the incoming light. Moreover, the critical dimensions of a photoresist pattern are often affected by any varying of thickness in the film layer underneath the anti-reflection coating. Ultimately, critical dimensions of the photoresist pattern will not be uniform.